Pump means for hydraulic jacks



Oct. 4, 1960 w. WILKENLOH ETAL 2,954,740

PUMP MEANS FOR HYDRAULIC JACKS Filed Jan. 17,1955

Inventors.-

WILHELP? Mum 1 AND E IC JAER 2,954,740 Patented Oct. 4, 1960 ice PUMPMEANS FOR HYDRAULIC JACKS Wilhelm Wilkenloh, Duisburg-Buchholz, andErich .liiger, Duisburg-Wanheim, Germany, assignors to RheinstahlWanheim Gesellschaft mit beschrankter Haftung Filed Jan. 17, 1955, Ser.No. 482,335

Claims priority, application Germany Jan. 16, 1954 "Claims. (Cl.103-196) The present invention relates to hydraulic arrangements adaptedto be used as props or jacks. For example, the structure of the presentinvention may be used as a prop extending between the floor and roof ofa mine. It is often desirable with such hydraulic arrangements toprovide very large forces which are difficult to obtain with knownhydraulic arrangements which are used as jacks or props, and where arapid movement of such a jack or prop into an expanded position isrequired it is necessary to use undesirably large pistons. Furthermore,with the known hydraulic arrangements a great deal of energy used inopcrating pumps of such arrangements is wasted.

One of the objects of the present invention is to provide a hydraulicjack or prop with a pump which enables energy applied to the pump duringone of its strokes but not used for that stroke to be stored in the pumpand automatically used as part of the force for effecting the succeedingstroke of the pump.

Another object of the present invention is to provide a hydraulic jackor prop with a pump which requires substantially equal forces for itssuction and pressure strokes.

A further object of the present invention is to provide a hydraulic jackor prop capable of being rapidly expanded without having undesirablylarge pistons.

An additional object of the present invention is to provide in anarrangement of the above type a pump structure capable of being manuallyoperated and being made up of simple and ruggedly constructed parts manyof which perform multiple functions.

With the above objects in view the present invention mainly consists ofan expandable and collapsible hydraulic arrangement to be used as a jackor a prop, this arrangement including a plurality of telescoped tubesand pump means in the tubes for expanding the same. An operating meansis connected to the pump means for moving the same through operatingcycles each of which includes a pressure stroke and asuction stroke, anda means, forming part of the pump means, is provided for storing energyapplied to the pump means through the operating means during one of thestrokes and applying the stored energy to the pump means during theother of the strokes.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantage thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawing.

The drawing is a longitudinal; central section through a hydraulicarrangement. constructed in accordance with the present invention, partof the structure being schematically illustrated, and parts of theelongated tubes being broken away so as to enable the structure to beconveniently illustrated.

Referring now to the drawing, it will be seen that an outer tube 6 hasan inner tube 3 telescopically slidable therein. The outer tube 6 isprovided at its bottom'end with an end wall 6. The inner tube 3 isprovided within the tube 6 with an end wall in the form of a piston 2,which slidably engages the inner surface of the tube 6. The end wall 6and end wall 2 form between themselves within the tube 6 a chamber 5.

A pair of pins 30 are fixed to the inner tube 3 and extend into theinterior thereof where they support an enclosure 8 having a bottom endwall formed with a bore passing therethrough. A hollow shaft 4 extendsthrough a bore of piston 2 and through the bore of the end wall ofenclosure 8 so that the interior 7 of enclosure 8 communicates throughthe shaft 4- with the chamber 5. The chambers 5 and 7 together with theinterior of shaft 4 form a high pressure chamber, as will be apparentfrom the description which follows. The lower end portion of the hollowshaft 1 extends slidably through a sealing ring 9 located in the bore ofpiston 2, and the top endof the shaft 4 is fixed to a tubular piston 25which extends slidably through a sealing ring 19 carried by theenclosure 8 at the bore thereof. This enclosure 8 also forms a housingfor a valve, as will be apparent from the description which follows.

A crank 12 is turnable within a tubular member 12 extending through andcarried by the wall of the inner tube 3, and this crank 12 has at itsouter end face a non-circular projection 11 to which a crank handle maybe fixed so that crank 12 may be manually rotated. A crank pin 13 isfixed eccentrically to the inner face of crank 12, and this pin 13extends into a recess within a block 14 fixed to the shaft 4 at theexterior thereof. The recess of block 14 into which the pin 13 extendsis large enough to permit rotation of the crank 12 while retaining thepin 13 in engagement with the block 14. Thus, during rotation of thecrank 12 the shaft 4 will be raised and lowered.

A piston 15 is fixed to the shaft 4 and thus moves therewith. A secondpiston 17 is located about the shaft 4 and is formed with a bore 15 intowhich the piston 15 slidably extends when the piston 15 is moved withrespect to piston 17 downwardly beyond the position shown in thedrawing. The bore 16 is a suction bore, andthe piston 17 is locatedslidably Within a cylindrical extension 1 of the' piston 2. A sealingring 18 is located on piston 17 at bore 15 to seal the engagementbetween pistons 15 and 17. Thus, when the crank 12 is turned to move theshaft 4 downwardly, as viewed in the drawing, the piston 15 will movedown and close the bore 16 so as to shut the interior of the cylindricalextension 1.

A ring 19 is located freely about the shaft 4- for movement with respectto the same and rests against the piston 15 in the position of the partsshown in the drawing. A spring 20 located about the shaft 4 has one endin engagement with a ring 21 which is fixed rigidly to the shaft 4 as bywelding or the like. Upon downward movement of the piston 15 with theshaft 4, the ring 19 will engage the piston 17 and there will besufiicient force in the spring 21} to urge the piston 17 downwardlytogether with the piston 15 as long as the pressure of the fluid mediumwithin the extension 1 is below a given value.

A plate 22' is retained against the bottom face of piston 2 and isformed with a central bore about which a coil spring 22 is located onthe upper face of the plate 22. This coil spring 22 bears against a ring9' which in turn bears against the sealing ring 9 to maintain the latteragainst the bottom face of the inner circular portion 2 of piston 2which. directly surrounds the shaft 4 and which is formed with borespassing therethrough, as shown in the drawing. Upon movement of pistons15 and 17 toward the piston 2, the pressure of the fluid in theextension 1 increases sufliciently to shift the sealing ring 9downwardly against the force of spring 22 so as to open the passagebetween chamber and extension 1 and the fluid in extension 1 thus flowsinto the chamber 5 during the downward pressure stroke of pistons and 17to effect upward displacement of piston 2 and tube 3. Because of thelarge volume displaced by pistons 15 and 17 during their downwardpressure stroke, as described above, a considerable quantity of fluidwill flow into the chamber 5 to effect a rapid expansion of the tubes,so that when the structure is used as a mine prop the top of tube 3 maybe quickly moved into engagement with the roof. Up to the present timeextremely large pistons have been used in order to obtain the necessaryrapid expansion of such a prop.

During the upward movement of shaft 4 caused by continued rotation ofcrank 12, the suction stroke of the pump takes place, and pistons 15 and17 move upwardly together, piston 17 being moved up by the spring 23which acts through the ring 24 on the. pistonv 17. Of course, the springis stronger than the spring 23 so as to be capable of moving the piston17 downwardly during its pressure stroke. During the upward movement ofthe shaft 4, the piston 17 moves up under the influence of spring 23until it engages the snap ring 17 located in a groove formed in theinner face of the extension 1. The upward movement of the pistons 15 and17 creates a partial vacuum within the cylindrical extension 1, andafter piston 17 engages the ring 17' the continued upward movement ofpiston 15 together with the shaft 4 opens the bore 16 so that fluid willflow from the interior of tube 3 through the bore 16 into the extension1.

It will be noted that because the chamber 5 communicates through shaft 4with the chamber 7, the pressure in these chambers is the same, and theupward movement of the shaft 4 moves the piston into the chamber 7 so asto increase the pressure of the fluid therein, and this increasedpressure acts to store energy which urges the piston 25 downwardly tocontribute to the force required for the succeeding pressure stroke.Thus, during the succeeding pressure stroke the force required by theoperator is diminished by the force of the fluid in the chamber 7 actingon the piston 25.

This operation will continue untfl the pressure required to move fluidfrom extension 1 into chamber 5 is so great that the spring 20 can nolonger move the piston 17 together with the piston 15 during thedownward stroke of the latter. At this time the smaller piston 15 worksby itself since the piston 17 remains in its upper position shown in thedrawing.

It will be noted that because the piston 17 no longer moves downwardlywith the piston 15, the continued downward movement of the latter withthe shaft 4 results in further compression of the spring 20 withoutmovement of the piston 17. However, this energy which is stored into thespring 20 is not wasted since it is expended in the succeeding suctionstroke by contributing to the force required for moving the shaft 4together with the piston 25 upwardly. Thus, with the above describedarrangement by proper choice of the sizes of pistons 25, 15, and 17 notonly is it possible to store during one stroke of the pump energy whichis used in the succeeding stroke of the pump, but also it is possible tomake the force required for the suction strokes substantially equal tothat required for the pressure strokes.

Since the force required to move the piston 25 into the chamber 7 isused in the succeeding pressure stroke the output of the pump is almostdouble so that with proper choice of the diameters of the pistons thisadvantage may be applied either toward obtaining a very large force withthe hydraulic arrangement of the invention or toward obtaining a verylarge speed of operation.

The enclosure 8 is formed in its top wall with a bore through which avalve body 31 slidably extends. A spring 32 in the enclosure 8, urges aring 33 against a sealing ring 34 which engages the valve body 31 andseals the engagement between the valve body 31 and the enclosure 8. Thevalve body 31 is itself axially bored and a valve member 35 formed withan outer axially extending groove 36 is slidable in the bore of valvebody 31. The bottom end of valve member 35 is enlarged and bears againstthe bottom end of the bore of body 31 to close this bore. A plurality ofring-shaped dish springs 37 engage a nut 38 to urge the ,bottom end ofvalve 35 against its seat. A cylindrical member 39 is freely locatedabout the enclosure 8 and is formed with elongated cutouts through whichthe pins 30 freely extend. The cylindrical member 39 has a top wallformed with a bore through which the body 31 extends, and this body 31is formed with an outer annular projection 40 against which the top wallof the member 39 is urged by the ring-shaped dish springs 41 locatedabout shaft 4 and between enclosure 8 and the bottom wall of thecylindrical member 39. A block 42 is located within the tube 3 above thevalve body 31 and is provided with a bottom extension 43 located inalignment with the valve 35. A spring 44 engages the top face of member39 and block 42 to urge the latter upwardly, and the top end of block 42is formed as a valve 45 which engages the annular member 46 located inthe interior of tube 3 to close the interior of tube 3, this annularmember 46 having its central opening in communication with a bore 47formed in the top wall of the tube 3. A crank 48 is turnably carried ina tube 49 fixed to and extending through the wall of the tube 3, and thecrank 48 has a non-circular extension 50 to which a handle is adapted tobe connected. An eccentric crank pin 51 is fixed to and extends from theinner face of crank 48 and extends into a recess of block 42, thisrecess having suflicient clearance to permit turning of crank 48 throughan angle sufiicient to open the passage through the annular member 46 aswell as to move the valve 35 downwardly and open the passage through thevalve body 31.

The interior of the tube 3 is filled with any suitable hydraulic fluidsuch as oil by turning the crank 48 so as to open the passage throughthe ring 46. Upon release of the crank 48, the spring 44 closes thispassage by moving the valve 45 into engagement with the ring 46. Also,during filling of the interior of tube 3 the crank 48 is turnedsufiiciently to cause extension 43 to open the valve 35 and the oilflows into chamber 7 and down the shaft 4 into chamber 5, the valve 35being returned to its closed position by springs 37 when the crank 48 isreleased.

During operation of the pump oil flows from the interior of tube 3 intothe extension 1 and from the latter into the chamber 5. Thus, in thisivay oil becomes displaced from the interior of tube 3 to the interiorof the tube 6 and the tubes thus expand whereby, when the prop is usedin a mine the top plate of the inner tube 3 is pressed with increasingpressure against the roof of the mineshaft.

To prevent increase of this pressure over a predetermined maximumamount, which would cause the tube 3 to buckle, the valve 31, 35 willact as a safety valve when the predetermined maximum pressure issurpassed. In this case the increased pressure in chamber 7 will liftthe valve body 31. Since this valve body 31 contacts .with its flange 40the member 39, this member will be also lifted against the force of thespring plates 41. The action will take place only when the pressure inthe chamber 7 is increased so that this pressure will overcome the forceof spring plates 41. When the valve body 31 is thus lifted the upper endof the valve stem 35 will abut against the extension 43 of the block 42and the valve will open so that compressed fluid may pass from thechamber 7 into the tube 3. Thereby the pressure in the chamber 7 and thechamber 5 connected thereto is reduced to the permissible maximumpressure.

When it is desired to move tube 3 back into the tube 6, the crank 48 isagain turned so as to move the block 42 downwardly, as viewed in thedrawing, and this results on the one hand in opening of the passagethrough ring 46 so that the interior of tube 3 communicates with theatmosphere, and on the other hand in the opening of the valve 35 so thatfluid in the chamber 5 may flow upwardly through the tube 4 into thechamber 7 and from thelatter through the valve body 31 into the tube 3.The valve 31, 35 has therefore two purposes. It will serve as outletvalve whenever it is desired to collapse the expanded prop, in whichcase the valve is operated by the crank 48. Secondly, the valve 31, 35will act as safety valve to prevent increase of the pressure fluid inthe chamber 7 above a predetermined maximum amount.

The hydraulic prop as described above operates as follows:

During the first part of the operation of the prop in which the fluidserves only to lift the deadweight of the inner tube 3 and the part ofthe mechanism connected thereto the piston 15 fixedly attached to thehollow shaft 4 will, during the downward movement of the shaft 4, closethe bore 16 in the larger piston 17 and compress the fluid in thechamber 1. Thereupon the valve 9 opens against the pressure of thespring 22 and the fluid will pass from the chamber 1 into the expansionchamber 5 and from there through the hollow shaft 4 also in the upperchamber 7. During the further downward stroke of the shaft 4 the plate19 pressed by the spring 20 against the upper surface of the piston 15will contact the upper surface of the piston 17 so that the two pistonsarefl coupled together and moved together downwardly, whereby a greatervolume of fluid is moved from the chamber 1 into the chambers 5 and 7.During the upward or suction stroke of the shaft 4 the pistons 15 and 17will move first together in upward direction under the action of thespring 23 until the upper surface of the piston 17 strikes the snap ring17. Thereupon the upward movement of the piston 17 stops and the piston15 will therefore move relative to the piston 17 further in upwarddirection opening again the bore 16 so that thefluid in the tube 3 maycommunicate with the chamber 1. It is understood that the spring 22 willclose the valve 9 during the upward movement of the pistons 17 and 15 sothat the compressed fluid in chambers 5 and 7 cannot pass into thechamber 1 during the upward suction stroke of the pump.

During this first part of the operation in which the prop is not underpressure the pistons 15 and 17 act together to displace a large amountof fluid during the pumping stroke and to provide thereby for a rapidexpansion of the prop.

When the top plate of the inner tube 3 abuts against the roof ofmineshaft, or against any other object to be supported, the rapidexpansion of the prop will stop and the fluid pressure in the chambers 5and 7 will increase with every pump stroke. During this part of theoperation the action of the mechanism will differ from the action asdescribed above. During the downward movement' of the shaft 4, that isduring the pumping stroke, the piston 15 will again close the bore inthe piston 17, compress the fluid in the chamber 1, the valve 9 willopen and the fluid compressed in the chamber 1 will pass therefrom intothe chambers 5 and 7. When the valve 9 opens, the pressure in thechamber 1 is of course the same as in the chambers 5 and 7 and thispressure will increase rapidly when the end plates of the tubes 3 and 6re.-

spectively are in engagement with the faces of the mine shaft. When thebuilt-up pressure is greater than the force necessary to compress thespring 20, and when the plate 19 contacts during the downward stroke ofthe shaft. 4 the upper surface of the piston 17, the piston 17 will dueto the increased pressure in the chambers 1, 5 and 7 remain stationaryand the spring 20 will be compressed between the plate 19 resting on thestationary piston 17 and the plate 21 fixed to the shaft 4. During thiscompression of the spring 20 energy will be stored in this spring, whichwill be released on the return stroke or suction stroke of the shaft 4.

It should be noted that during the downward pumping stroke the fluidpressure in the chambers 5 and 7 acts on the top surface of the piston25 and this force acts therefore during the downward stroke in the samedirection as the force on the crank 12. During the pumping stroke thepiston 15, after closing the bore in the piston 17 and opening the valve9 has to overcome the full pressure in the chamber 1. The force actingon the piston during this period is equal to the force exerted by thecrank 12 and I the force of the compressed fluid acting on the piston 25minus the force with which the spring 20 resists its compression.

During the upward suction stroke the piston 25 works against the fullpressure in the chamber 7. The force acting during this period on thepiston 25 is equal to the sum of the forces from the crank 12 and theforce which the spring 20 returns during its expansion, minus thenuegligible small force of the atmospheric pressure acting on the piston15 during its movement out of the chamber 1.

By properly dimensioning the spring 20 and the diameters of pistons 15and 25 it is possible to reduce the difference between the necessarycrank forces during the pumping and the suction stroke to a minimum. Anincrease of the diameter of piston 25 relative to the diameter of piston15 will reduce the necessary crank force during the downstroke and willrequire an increased crank force during the up stroke.

A pump action is always assured by the two pistons 15 and 25. At the endof the up stroke the chamber 1 communicates with the interior of tube 3and fluid will enter from the tube 3 into the chamber 1. During the downstroke the piston 15 inters into the chamber 1 and displaces fluidtherefrom into the chambers 5 and 7. Every down stroke thereforeincreases the amount of fluid in the pressure chambers 5 and 7. Duringthe up stroke the amount of liquid in the chambers 5 and 7 remainsconstant and the pressure increases as the piston 25 displaces liquidfrom the chamber 7 into the chamber 5 whereby the top plate of tube 3will be pressed with increased force against the wall it abuts. Itshould be noted that the energy accumulated by and returned to thepiston 25 is increased with increasing pressure of the fluid in thechambers 5 and 7. The accumulation of energy therefore increases as thenecessary force for the pumping action becomes greater. A compensationof the crank forces during the up and down strokes therefore takes placeduring all pressure conditions in the chambers 5 and 7.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofhydraulic props or jacks differing from the types described above.

While the invention has been illustrated and described as embodied inpump arrangements for hydraulic props or jacks, it is not intended to belimited to the details shown, since various modifications and structuralchanges may be made without departing in any way from the spirit of thepresent invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of'prior art, fairly constitute essentialchar- 7 acteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. In a pump and pressure chamber unit, in combination, a first pressurechamber; a second pressure chamber; reciprocating pump means having apressure stroke and a suction stroke for pumping pressure fluid intosaid first pressure chamber during said pressure stroke thereof andincluding a hollow shaft communicating at opposite ends thereof withsaid first and said second pressure chamber, respectively; a pistonfixedly mounted on said hollow shaft for movement in and out of saidsecond pressure chamber for increasing the volume of said secondpressure chamber during the pressure stroke of said pump means and fordecreasing the volume of said second pressure chamber during the suctionstroke of said pump means; and operating means operatively connected tosaid hollow shaft for reciprocating said pump means and for moving saidpiston out of said second pressure chamber during the pressure stroke ofsaid pump means and into said second, pressure chamber during thesuction stroke of said pump means.

2. In a pump and pressure chamber unit, in combination, a first pressurechamber; a second pressure chamber; reciprocating pump means having apressure stroke and a suction stroke for pumping pressure fluid intosaid first pressure chamber during said pressure stroke thereof andincluding a cylinder having an end wall, piston means arranged in saidcylinder movable towards and away from said end wall, inlet valve meansformed in said piston means, outlet valve means in said end wall, and ahollow shaft communicating at opposite ends thereof with said first andsecond pressure chamber, respectively, and operatively connected to saidpiston means; a piston fixedly mounted on said hollow shaft for movementin and out of said second pressure chamber for increasing the volume ofsaid second pressure chamber during the pressure strokes of said pumpmeans and for decreasing the volume of said second pressure chamberduring the suction stroke of said pump means; and operating meansoperatively connected to said hollow shaft for reciprocating said pumpmeans and for moving said piston out of said second pressure chamberduring the pressure stroke of said pump means and into said secondpressure chamber during the suction stroke of said pump means.

3. In a pump and pressure chamber unit, in combination, a first pressurechamber; a second pressure chamber; reciprocating pump means having apressure stroke and a suction stroke for pumping pressure fluid intosaid first pressure chamber during said pressure stroke thereof andincluding a cylinder having an end wall, piston means arranged in saidcylinder movable towards and away from saidend wall, inlet valve meansformed in said piston means, outlet valve means in said end wall, and ahollow shaft communicating at opposite ends thereof with said first andsecond pressure chamber, respectively, and operatively connected to saidpiston means, said hollow shaft being concentrically arranged with saidoutlet valve means and being guided therein; a piston fixedly mounted onsaid hollow shaft for movement in and out of said second pressurechamber for increasing the volume of said second pressure chamberduringthe pressure stroke of said pump means and for decreasing thevolume of said second pressure chamber during the suction stroke of saidpump means; and operating means operatively connected to said hollowshaft for reciprocating said pump means and for moving said piston outof said second pressure chamber during the pressure stroke of said pumpmeans and into said second pressure chamber during the suction stroke ofsaid pump means.

4. In a pump and pressure chamber unit, in combination, a first pressurechamber; a second pressure chamber;

. 8 reciprocating pump means having a pressure stroke and a suctionstroke for pumping pressure fluid into said first pressure chamberduring said pressure stroke thereof and including a cylinder having anend wall formed with a bore therethrough communicating at one endthereof with said first pressure chamber, a hollow shaft communicatingat opposite ends thereof with said first and second pressure chamber,respectively, and extending with clearance through said bore, combinedseal and non-return valve means located in said bore about said shaftfor preventing fluid from flowing from said first pressure chamber intosaid cylinder and permitting fluid to flow through said bore from saidcylinder into said first pressure chamber, a first piston slidable insaid cylinder and formed with a second bore through which said shaftpasses with clearance, a second piston fixed to said shaft and slidableinto and out of said second bore, and spring means operatively connectedto said shaft and engaging said pistons for moving said first pistontogether with said second piston when pressure in said cylinder is belowa predetermined value; a third piston fixedly mounted on said hollowshaft for movement in and out of said second pressure chamber forincreasingthe volume of said second pressure chamber during the pressurestroke of said pump means and for decreasing the volume of said secondpressure chamber during the suction stroke of said pump means; andoperating means operatively connected to said hollow shaft forreciprocating said pump means and for moving said third piston out ofsaid second pressure chamber during the pressure stroke of said pumpmeans and into said second pressure chamber during the suction stroke ofsaid pump means.

5. In a pump and pressure chamber unit, in combination, a first pressurechamber; a second pressure chamber; reciprocating pump means having apressure stroke and a suction stroke for pumping pressure fluid intosaid first pressure chamber during said pressure stroke thereof andincluding a cylinder having an end wall formed with a bore therethroughcommunicating at one end thereof with said first pressure chamber, ahollow shaft communicating at opposite ends thereof with said first andsecond pressure chamber, respectively, and extending with clearancethrough said bore, combined seal and nonreturn valve means located insaid bore about said shaft for preventing fluid from flowing from saidfirst pressure cham ber into said cylinder and permitting fluid to flowthrough said bore from said cylinder into said first pressure chamber, afirst piston slidable in said cylinder and formed with a stepped borehaving a smaller diameter at the end of the bore directed toward saidcylinder through which said shaft passes with clearance, a second pistonfixed to said shaft and having an outside diameter equal to said smallerdiameter of said here, said second piston being movable from a positionin which it is located outside said smaller diameter of said bore sothat fluid may pass between said first and second piston into saidcylinder and a position in which said second cylinder is at least partlylocated within said smaller diameter of said stepped bore therebypreventing passage of fluid through said stepped bore, and spring meansfixedly connected at one end thereof to said shaft and engaging saidfirst piston with the other end thereof, for moving, during the pressurestroke, said first piston together with said second piston when pressurein said cylinder is below a predetermined value, while said firstcylinder remains stationary during the pressure stroke when saidpredetermined value is surpassed, said spring means being stressedduring said pressure stroke when said first piston remains stationary sothat energy is stored in said spring means and this stored energy isreturned during the suction stroke of said pump means; a third pistonfixedly mounted on said hollow shaft for movement in and out of saidsecond pressure chamber for increasing the volume of said secondpressure chamber during the pressure stroke of said pump means and fordecreasing the volume of said second pressure chamber during the suctionstroke of said pump means; and operating means operatively connected tosaid hollow shaft for reciprocating said pump means and for moving saidthird piston out of said second pressure chamber during the pressurestroke of said pump means and into said second pressure chamber duringthe suction stroke of said pump means.

430,251 Kendall June 17, 1890 10 Wakefield Jan. 4, 1910 Gathmann Jan.25, 1910 Brian Feb. 1, 1916 Bronson Apr. 6, 1926 McNab Oct. 27, 1931Bohnenblust July 21, 1936 Schwerin Feb. 3, 1948 Page Apr. 20, 1948Phenning Apr. 24, 1951

